I need a dictionary for a custom inspector

So I need a dictionary structure to save things in a custom editor. I know that dictionaries are not allowed so I tried to create my own type, finding out that neither generics are allowed. But List is. So starting from this I have created a class that extends the List and add a small functionality that allows me to treat this object as a Dictionary. Here's the code:

 public class CustomDictionary : List<CustomDictionaryItem>
 {
     public void Add (string key, UnityEngine.Object value)
     {
         this.Add (new CustomDictionaryItem (key, value));
     }

     public bool ContainsKey (string key)
     {
         return this.Any (sdi => sdi.Key == key);
     }
     
     public bool Remove (string key)
     {
         return this.RemoveAll (sdi => sdi.Key == key) != 0;
     }

     public UnityEngine.Object this [string key] {
         get {
             if (ContainsKey (key))
                 return (UnityEngine.Object)this.First (sdi => sdi.Key == key).Value;
             return null;
         }
         set {
             if (ContainsKey (key)) {
                 CustomDictionaryItem item = this.First (sdi => sdi.Key == key);
                 item.Value = value;
             } else
                 Add (key, value);
         }
     }

     public List<string> Keys {
         get {
             return this.Select (sdi => sdi.Key).ToList ();
         }
     }

     public List<UnityEngine.Object> Values {
         get {
             return this.Select (sdi => (UnityEngine.Object)sdi.Value).ToList ();
         }
     }
 }

 [Serializable]
 public class CustomDictionaryItem
 {
     [SerializeField]
     private string m_key;

     public string Key{ get { return m_key; } set { m_key = value; } }

     [SerializeField]
     private UnityEngine.Object m_value;

     public UnityEngine.Object Value{ get { return m_value; } set { m_value = value; } }

     public CustomDictionaryItem (string key, UnityEngine.Object value)
     {
         m_key = key;
         m_value = value;
     }
 }

I would like it to be typed to be honest(instead of UnityEngine.Object) but I tried to remove the generics in hope that it would work. It doesn't. The values are not saved after the game is started.

PS: this "public List m_customDictionary;" works ok, as it should, but does not have the functionality that I want. Any ideas? Thank you!